What is the ruling on marriage if the woman waived her Mahr (dower) before the marriage contract, and the husband agreed, and the marriage took place with a guardian and two witnesses, and they were blessed with children?

If the marriage is conducted with a guardian and witnesses, it is valid, and the omission of naming the dower therein does not harm it. The wife is entitled to a dower equivalent to that of her peers (mahr al-mithl), and her waiver of the dower before it becomes an obligation upon the husband is not considered. However, she may waive it after it has become an obligation.